Umbraco path indexed properly. The only useful/interesting code here is really at the end of line 83. :)

PathIndexer.cs

using System.IO;
using Examine;
using Examine.LuceneEngine;
using Examine.LuceneEngine.Indexing;
using Lucene.Net.Analysis;
using Lucene.Net.Util;
using Umbraco.Core.Composing;
using Constants = Umbraco.Core.Constants;

// ReSharper disable once CheckNamespace
namespace MySite.Web.AppStart
{
    public class IndexComposer : ComponentComposer<IndexComponent>
    {
    }

    public class IndexComponent : IComponent
    {
        private readonly IExamineManager examineManager;

        public IndexComponent(IExamineManager examineManager)
        {
            this.examineManager = examineManager;
        }

        public void Initialize()
        {
            if (!examineManager.TryGetIndex(Constants.UmbracoIndexes.ExternalIndexName, out var index))
            {
                return;
            }

            var coll = ((Examine.LuceneEngine.Providers.LuceneIndex) index).FieldValueTypeCollection;
            
            index.FieldDefinitionCollection.AddOrUpdate(new FieldDefinition("path", "csv"));
            coll.ValueTypeFactories.AddOrUpdate("csv", new DelegateFieldValueTypeFactory(s => new FullTextType(s, new WhitespaceAndCommaAnalyzer())));

            //index.FieldDefinitionCollection.
        }

        public void Terminate()
        {
        }
    }

    public class WhitespaceAndCommaAnalyzer : Analyzer
    {
        public override TokenStream TokenStream(string fieldName, TextReader reader)
        {
            return (TokenStream)new WhitespaceAndCommaTokenizer(reader);
        }

        public override TokenStream ReusableTokenStream(string fieldName, TextReader reader)
        {
            Tokenizer tokenizer = (Tokenizer)this.PreviousTokenStream;
            if (tokenizer == null)
            {
                tokenizer = (Tokenizer)new WhitespaceAndCommaTokenizer(reader);
                this.PreviousTokenStream = (object)tokenizer;
            }
            else
                tokenizer.Reset(reader);
            return (TokenStream)tokenizer;
        }
    }

    public class WhitespaceAndCommaTokenizer : WhitespaceTokenizer
    {
        public WhitespaceAndCommaTokenizer(TextReader @in) : base(@in)
        {
        }

        public WhitespaceAndCommaTokenizer(AttributeSource source, TextReader @in) : base(source, @in)
        {
        }

        public WhitespaceAndCommaTokenizer(AttributeFactory factory, TextReader @in) : base(factory, @in)
        {
        }

        protected override bool IsTokenChar(char c)
        {
            return base.IsTokenChar(c) && !','.Equals(c);
        }
    }

}